Sonde locator

ABSTRACT

In order to locate an inaccessible object such as an underground boring tool, the inaccessible object having means, for example a solenoid, to generate a magnetic field, a locator is provided with a detector for detecting the generated magnetic field. The detector may be an aerial array of three mutually perpendicular coils. When the locator is moved from a first position to a second position errors may arise in the measurements due to misalignment of the locator. In order to minimise such errors, a magnetic compass is provided on the locator to ensure that the locator can be maintained in the same position relative to the Earth&#39;s magnetic field. The magnetic compass is not affected by the magnetic field generated by the solenoid as this magnetic field is an A.C. magnetic field.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a locator for determining the positionof a sonde. Such a sonde may, for example, be mounted on a boring tooland may generate electromagnetic signals which enable the position ofthat boring tool to be determined by the locator.

2. Description of Related Art

In WO 96/29615 corresponding to U.S. Pat. No. 5,917,325, the disclosureof which is incorporated herein by reference, we discussed anarrangement for locating an underground boring tool in which a magneticfield generated by a solenoid was detected at two measuring locations.Use may be made of the relationship between the axial and radial fieldsof the solenoids, and a tilt sensor was included to indicate if the axisof the solenoid was not horizontal. The solenoid and tilt sensor maythen be incorporated in a sonde. In general, the locator uses an aerialarray to measure field components in three mutually perpendiculardirections. Using amplitude and phase data from each aerial array, thelocation of the sonde, and hence the boring tool, can be determined.

Thus, as shown in FIG. 1, an underground boring tool 10 has a sonde 11which contains a solenoid which generates a magnetic field, with theaxis 12 of the solenoid being aligned with the longitudinal axis of theboring tool 10. The axis 12 will be the direction of movement of theboring tool, unless the boring tool is diverted by ground variation orobstacles, or by deliberate steering action. A locator having an aerialarray comprising three coils X₃ Y₃ and Z₃ will, when placed directlyabove the sonde 11 and in alignment with it so that the Y₃ coil isparallel to the axis 12, the Z₃ coil is perpendicular to the axis in away which makes that coil Z₃ vertical when the axis 12 is horizontal,have a maximum signal in the direction axis 12 with the X₃ and Y₃ coilsdetecting a null field. Any diversions from this position will result insignals from the X₃ and Z₃ coils, which signals will vary in amplitudeand phase according to the extent and direction of such divergence. Asimilar arrangement will apply if the locator is positioned ahead of thesonde 11 although the signal magnitude will then be reduced due todistance. In tracking a boring tool, it is common to position a locatorahead of the tool in the anticipated direction of its motion.

However, if the locator is not correctly aligned, an incorrectmeasurement will result. Whilst the operator may endeavour to maintainthe orientation of the locator relative to the boring tool 10, this isnot always reliable since the boring tool cannot be seen.

SUMMARY OF THE INVENTION

A locator is provided with a detector for detecting the magnetic fieldgenerated by, for example, a solenoid in an inaccessible object whichmight be an item such as an underground boring tool. The detectorpreferably comprises an aerial array of three mutually perpendicularcoils for detecting the generated magnetic field. When the locator ismoved from a first position to a second position errors may arise in themeasurement due to misalignment of the locator. In order to minimizesuch errors, a magnetic compass is provided on the locator to insurethat the locator can be maintained in the same position relative to theearth's magnetic field. The magnetic compass is not affected by themagnetic field generated by the solenoid as this magnetic field is anA.C. magnetic field.

BRIEF DESCRIPTION OF THE DRAWINGS

An embodiment of the present invention will now be described in detail,by way of example, with reference to the accompanying drawings, inwhich:

FIG. 1 shows fields generated by a solenoid-containing sonde, and hasalready been discussed; and

FIG. 2 shows location of a boring tool using a locator according to thepresent invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to FIG. 2, an underground boring tool 10 has a sonde with asolenoid generating a magnetic field, the axis of the solenoid beingshown at 12. This arrangement is similar to that shown in FIG. 1 andcorresponding components will be indicated by the same referencenumerals. In order to achieve satisfactory location of the boring tool10 using a locator 20, the operator seeks to place the locator 20directly ahead of the boring tool 10 at a position B. In order to dothis, it is usual for the operator first to place the locator 20slightly to one side of the expected line of the axis 12, at position B2in FIG. 2 and then move the locator 20 until it coincides with the axis12, on the basis of amplitude and phase measurements from the coils ofthe locator 20. The coils are as in FIG. 1, and will not be described inmore detail now.

If a locator 20 a is mis-aligned with the axis 12, and moved toward theaxis to a position B1, measurements is taken by the locator 20 a may beincorrect.

Therefore, in accordance with the present invention, the locator 20 hasthereon a magnetic compass 21. The compass points NS, and so defines theorientation of the locator 20 relative to the Earth's magnetic field.Therefore, when the locator 20 is moved from one position to another,e.g. from a position over the boring tool 1 to the position B2 in FIG.2, the locator can ensure that the compass orientation remainsunchanged, so that the locator 20 has the same orientation relative tothe Earth's magnetic field. Thus, mis-alignments such as shown atposition B1 in FIG. 2 can be avoided. Hence, incorrect measurements maybe avoided. Note that the compass 21 is unaffected by the magnetic fieldfrom the sonde 2, which field is shown schematically by dotted lines,because such a field is an A.C. field.

At its simplest, the magnetic compass 21 may be a simple compass ofconventional type, such as those used by walkers, since all that isneeded is for the operator of the locator to have a visual indication ofthe direction to the boring tool. However, it is also possible to useother compass arrangements. In particular, the need to record a compassbearing at the initial position can be obviated if the compass isprovided with an adjustable pointer or marked bezel, which can be set tomatch the north point of the needle, in a similar manner to the settingof a barometer pointer to its last reading. By turning the locator untilthe needle and adjustable mark correspond, the operator then knows thatthe locator has its original orientation.

Other arrangements such as magnetometer or ‘solid-state’ compasses orgyroscopic devices might find application in some circumstances.

What is claimed is:
 1. A method for detecting the relationship between alocator and an inaccessible object, said inaccessible object havingmeans for generating a magnetic field and said locator having detectingmeans for detecting the generated magnetic field and a magnetic compass,the method comprising: (a) measuring horizontal and vertical componentsof said generated magnetic field using the locator in a first position;(b) processing said measurements to determine the separation of saidinaccessible object from said locator in the first position; (c) movingsaid locator to a second position and using said compass to ensure thatsaid locator has the same orientation relative to the Earth's magneticfield as in said first position; (d) measuring horizontal and verticalcomponents of said generated magnetic field using the locator in thesecond position; and, (e) processing said measurements to determine theseparation of said inaccessible object from said locator at the secondposition.
 2. A method according to claim 1 wherein said generatedmagnetic field is an A.C. magnetic field.
 3. A method according to claim1 wherein said magnetic compass has a display which is visible from theoutside of said locator.
 4. A method according the claim 1, wherein saidmeans for generating a magnetic field is a solenoid.
 5. A methodaccording to claim 1, wherein the inaccessible object is a boring tool.